Linux-Powered Amateur Rocket Goes USB

The next Portland State Aerospace Society rocket, scheduled for first launch this summer, will have new hardware, including a switch from CAN to USB.

In summer 2005, I stood on a sandy hill a couple miles east
of Bend, Oregon. Through my binoculars, I could see people scattered
in a distant ring around our 12-foot amateur rocket, waiting to take
pictures when it launched. A mile away, I could see the tents and cars
at ground control.

I was part of a recovery team for the Portland State Aerospace
Society (PSAS). PSAS is a completely open-source aerospace engineering
group. You can take our open-source software and open hardware designs
from our Web site (see Resources) and make your own rocket. Our long-term goal is to
guide our rocket into space actively and put a cube satellite into orbit.

That summer day, we weren't going into orbit; we were just testing our
latest rocket. Our rocket would launch, deploy its parachute at about
18,000 feet above the ground, and then drift safely to the ground,
all the while spewing sensor data over our 802.11 wireless telemetry
link. Once the rocket had landed, the recovery teams would use the GPS
coordinates to find the rocket.

Over my 2-meter ham radio, I could hear Andrew Greenberg (PSAS's
self-proclaimed “benevolent dictator”) warning the bystanders at the
launch site that the rocket motor was about to go live. The DTMF tones
to arm the rocket followed.

“...3...2...1. We have liftoff!” The ground crew could see the streaming
video from the rocket showing the ground become farther and farther away. The
Java RocketView software displayed the rocket's sensor data: GPS
coordinates, acceleration, rotation, pressure and the state of all the
rocket's subsystems. Everything looked good.

I watched the rocket get smaller and smaller as it shot into the sky. The
Linux flight computer on board the rocket would evaluate all the sensor
data and decide when to deploy the parachute. The parachute needed to be
deployed in the five-second window when the rocket reached its peak altitude
(apogee), slowed down and started to fall downward.

Figure 4. RocketView Screenshot (Photo Credit: Jamey Sharp)

At ground control, the crew watched the flight computer decide
to deploy the drogue shoot. Everyone cheered, because the hard part of
the flight was over. Or so we thought.

Five seconds later, the flight computer figured out that the rocket
was still falling. It tried to deploy the main parachute, but it was
still accelerating, as if the parachutes hadn't deployed. Something was
wrong. Andrew frantically began to send the DTMF tones to the rocket for
an emergency parachute deployment. The flight computer reported seeing
the DTMF tones, but the rocket continued to plummet toward the ground.

Thirteen seconds later, the link to the flight computer was dead. The
last known speed was more than 500mph, with a GPS reading about 1,000 feet
off the ground. The depressed ground crew relayed the last-known latitude
and longitude from RocketView.

Dave Allen, my fellow recovery team member, was eager to get to the
rocket first. Dave and I got as close to the GPS coordinates as we could
using the road and a four-wheel drive. Then we started hiking through
the desert.

Finally, I spotted a glint of metal in the middle of a scrub brush. About
a foot of rocket was sticking out of the ground. If we didn't have the
GPS coordinates, it would have been impossible to find.

Figure 5. Rocket Crash (Photo Credit: Sarah Sharp)

PSAS members showed up and we began to dig the rocket out. Our 12-foot
rocket had been compressed into a three-foot piece of twisted metal. The
electronics were dust and bits of broken silicon. Amazingly, Baker,
our sock monkey survived. He was a little squished, and his helmet was
ripped, but he would fly another day.